1. Field of the Invention
The present invention relates to a method for driving a simple matrix liquid crystal display device comprising a plurality of liquid crystal elements each being provided in correspondence with each pixel and a plurality of row electrodes and column electrodes for driving the liquid crystal elements by a super-twisted nematic system (hereinbelow, referred to as a STN system) wherein predetermined voltages are applied to the electrodes to control each liquid crystal element so as to produce brightness in response to an effective value of applied voltage whereby a predetermined picture image is displayed on a display area which is comprised of a matrix of the liquid crystal elements. In particular, the present invention relates to a method for driving a simple matrix liquid crystal display device which is capable of reducing voltage for driving the display device.
2. Discussion of the Background
Conventionally, as methods for driving a simple matrix liquid crystal display device provided with electrodes used commonly for a plurality of liquid crystal elements, there are a driving system based mainly on a so-called line successive driving system and a driving system based mainly on a multiple line addressing driving system (or it is called a MLA system).
The line successive driving system is a driving system in which predetermined voltages are successively applied to electrodes of every row, and at the same time, predetermined voltages are applied to a plurality of column electrodes whereby control voltages are applied to the row electrodes. Then, each of the liquid crystal elements is controlled to have a transmittance in response to an average effective voltage applied during a time in which the voltages are once applied to all the row electrodes (hereinbelow, referred to as a frame). A predetermined picture image is displayed for each frame period.
The MLA driving system is a driving system in which all the row electrodes constituting a display picture area are divided into subgroups each comprising a plurality of row electrodes (a simultaneously selected number), and predetermined voltages are applied to row electrodes for each subgroup and at the same time, predetermined voltages are applied to a plurality of column electrodes wherein the above-mentioned operation is repeated at least the same number of times as the simultaneously selected number to all the subgroups. Thus, each of the liquid crystal elements is controlled to have a transmittance in response to an average effective voltage applied during a time in which the above repetitive operations are finished (it is called a frame period), and a displayed picture image is formed in each frame period. Such MLA system is disclosed in Japanese Unexamined Patent Publication JP-A-6-27907, U.S. Pat. No. 5,262,881 and Japanese Unexamined Patent Publication JP-A-8-234164 and so on.
In the MLA driving system, when predetermined voltages are simultaneously applied to a plurality of row electrodes, the voltages applied to the column electrodes are the product of a unit column voltage and values obtained by performing calculation of a plurality of display data at the intersections of column electrodes and row electrodes and column data of orthogonal matrix used for applying the scanning voltages. The maximum value of magnifying power obtained by the matrix calculation suffers restriction by an orthogonal matrix used for the calculation, and it takes at most a value of the number of rows in the matrix.
The liquid crystal display device has been used as a display device for a man-machine interface with the progress of highly intelligent society. In recent years, it is widely used not only for a desktop type personal computer but also for a notebook type personal computer, PDA (a portable information terminal) or a portable telephone, which is suitable for carrying, taking an advantage of thin and light in weight. As a result, the development of the liquid crystal display device tends to increase the surface area of the screen as well as improvements in reduction of the weight and low power consumption.
In such liquid crystal display device, various measures have been taken to lower the power consumption rate. In more detail, there are measures to form a liquid crystal element capable of responding to a low effective voltage or to use a reflection type liquid crystal element without requiring a back light. Further, there is published xe2x80x9cGeneral-purpose addressing technology for an effective value response type liquid crystal display device (SID, a record of a meeting of SID international display research society 1988, p. 80-p. 85)xe2x80x9d as papers for reporting the relation between a driving method for such liquid crystal display device and electric power consumption. The papers report that when a multiple line driving is performed under conditions that L={square root over (M)} (where M represents the total number of row electrodes for a display area and L represents a simultaneously selected member) and the optimum bias ratio at which a ratio of an effective voltage versus a ratio between an effective voltage in an ON display time and an effective voltage in an OFF display time becomes the maximum is used, a driving voltage for the liquid crystal display device can be reduced in comparison with a case of using the line successive driving system.
The conventional liquid crystal display device uses a lithium ion battery (a button battery) of relatively high voltage (about 3.3 V) and reduced weight. However, the display device requires a driving voltage of 7-9 V even though an improvement of a liquid crystal material has been made, and accordingly, there is a voltage increase of about 3 times. As a result, there caused power loss due to a voltage increase circuit, which was against an attempt to lower consumption power. Thus, it was impossible to achieve the purpose of lowering consumption power to an extent of a sufficient utility. Further, since such voltage increase circuit required a fairly high breakdown strength, a generally utilized 5 V standard logic process device, which has generally been used, could not be used to increase a degree of integration. Accordingly, a logic process for inclusive use had to be developed to increase a degree of integration. As a result, there is resulted an increase of cost for the liquid crystal display device including a driving device and a prolonged term in designing. Further, there were problems of causing an additional cost in changing designing and difficulty in responding a demand of multi-item-small-production.
In forming actually the voltage increase circuit, there is a problem that the effective voltage changes due to the temperature dependence of liquid crystal whereby it is impossible to apply predetermined column voltages and row voltages. Accordingly, it is necessary to determine a voltage increase level in consideration of a temperature for the liquid crystal. However, working voltages to the liquid crystal itself are apt to vary under the above-mentioned condition. Accordingly, it was necessary to determine a voltage increase level with a larger margin so as to assure operating performance in a low temperature region. This created a cause of an increased power consumption rate by the provision of the voltage increase circuit as an addition circuit.
Further, besides the temperature dependence of the driving voltage, the response speed of liquid crystal becomes high in a high temperature region in a liquid crystal display for providing display data of static images, and there causes reduction of the contrast due to the frame response inherent to the passive matrix, whereby a phenomenon which deteriorates visibility takes place. Recently, there is a demand for a small or middle type liquid crystal devices having the performance capable of sequentially displaying cuts of images or scrolling of images of letters regardless of a high temperature region, which inevitably reduces visibility even in a static image with the tendency that liquid crystal material becomes quickly responsive. In order to respond such demands, it is necessary to increase a driving frequency for the liquid crystal device to thereby prevent a reduction of contrast ratio resulted from the frame response. However the power consumption rate of the liquid crystal display device tends to increase with an increase of the driving frequency. Such poor operational environments create a factor that the device does not have a sufficient utility.
As a result of extensive study to eliminate the above-mentioned problems, the inventors of the present invention have found that when a MLA driving is used against the reduction of visibility and the MLA driving is performed under a condition of Lxe2x89xa0{square root over (M)}, a voltage difference is produced between the maximum column voltage and row voltage at the optimum bias ratio at which an effective voltage ratio in an ON display time and an OFF display time becomes the maximum, and that in a graph having an abscissa which represents the ratio of an effective voltage in an ON display time to an effective voltage in an OFF display time and an ordinate which represents a driving voltage necessary to drive the liquid crystal elements, when one of the maximum column voltage and the scanning voltage is increased, the other is decreased wherein the maximum column voltage coincides with the scanning voltage at a bias ratio other than the maximum bias ratio, and at which point, the driving voltage becomes the minimum and there exists the minimum bias ratio which is lower than the driving voltage at the time of the optimum bias, and thus, the present invention has been accomplished.
It is an object of the present invention to provide a method for driving a simple matrix liquid crystal display device, which can reduce a power consumption rate in comparison with a driving method using the conventional MLA driving system while the ratio of an effective voltage in an ON display time to an effective voltage in an OFF display time can be assured.
Further, it is an object of the present invention to provide a method for driving a simple matrix liquid crystal display device, which can be driven practically by using a battery such as a button battery and which can increase a degree of integration for a driving circuit for the liquid crystal display device by using a standard logic process device.
In accordance with the first aspect of the present invention, a multiple line addressing driving is effected with an L number of simultaneously selected row electrodes to provide Lxe2x89xa0{square root over (M)} where M represents the total number of row electrodes for driving a display area and L represents the number of simultaneously selected row electrodes, wherein driving is performed at a bias ratio which is deviated toward the minimum bias ratio at which a driving voltage is the minimum with respect to the optimum bias ratio BOPT at which a ratio of an effective voltage value in an ON display time to an effective voltage value in an OFF display time is the maximum.
According to the second aspect of the present invention, in the above-mentioned first aspect, the display area is divided into subgroups each comprising L lines; column elements selected in an orthogonal matrix of L lines composed of +1 and xe2x88x921 are made corresponding to each line of the subgroups; row voltage levels where +1 corresponds to +VR and xe2x88x921 corresponds to xe2x88x92VR are applied to each row electrode of the subgroups; inner products are obtained from an L number of column data elements, having a value xe2x88x921 in an ON display time or +1 in an OFF time, which intersect a certain row electrode and column elements in the orthogonal matrix of L lines; predetermined column voltages in proportion to the inner products are applied to the column electrodes in synchronism with the row electrodes, and a bias ratio BX given by VR/VC where VC represents the maximum column voltage satisfies 1xe2x89xa6BX less than BOPT.
According to the third aspect of the present invention, in the first or second aspect, 0.3{square root over (M)}xe2x89xa6Lxe2x89xa62{square root over (M)} and 0.3BOPTxe2x89xa6BXxe2x89xa60.9BOPT are satisfied.
According to the fourth aspect of the present invention, in the first or second aspect, 40xe2x89xa6Mxe2x89xa6100 and BXxe2x89xa60.7BOPT are satisfied.
According to the fifth aspect of the present invention, in the first or second aspect, BX=1 is satisfied.
According to the sixth aspect of the present invention, in the first, second, third, fourth or fifth aspect, M=20-40 and L=4 are satisfied.
In accordance with the seventh aspect of the present invention, a multiple line addressing driving is effected with an L number of simultaneously selected row electrodes to provide {square root over ((M/Lxc2x7(L+D)))}xe2x89xa0N where M represents the total number of row electrodes for driving a display area, L represents the number of simultaneously selected row electrodes, D represents the number of dummy row electrodes and N represents the maximum magnifying power of a unit column voltage obtained by a predetermined matrix calculation based on display data and scanning voltages applied to the row electrodes, wherein driving is performed at a driving bias ratio which is deviated toward the minimum bias ratio at which a driving voltage is the minimum with respect to the optimum bias ratio BOPT at which a ratio of an effective voltage value in an ON display time to an effective voltage value in an OFF display time is the maximum.
According to the eighth aspect of the present invention, in the seventh aspect, the display area is divided into subgroups each comprising L lines; column elements selected in an orthogonal matrix of L+D lines composed of +1 and xe2x88x921 are made corresponding to each line of the subgroups; row voltage levels where +1 corresponds to +VR and xe2x88x921 corresponds to xe2x88x92VR are applied to each row electrode of the subgroups; an L number of column data elements intersecting a certain row electrode are represented as xe2x88x921 in an ON display time or +1 in an OFF time and a D number of dummy data are made corresponding to column data elements to prepare an L+D number of column data elements; inner products are obtained from such column data elements and column elements in the orthogonal matrix of L+D lines; predetermined column voltages in proportion to the inner products are applied to the column electrodes in synchronism with the row electrodes, L which satisfies {square root over ((M/Lxc2x7(L+D)))}xe2x89xa0N where N represents the maximum value of the inner products is used, and a bias ratio BX given by VR/VC where VC represents the maximum column voltage satisfies 1xe2x89xa6BX less than BOPT.
According to the ninth aspect of the present invention, in the seventh or eighth aspect, 0.3{square root over (M)}xe2x89xa6L+Dxe2x89xa62{square root over (M)} and 0.3BOPTxe2x89xa6BXxe2x89xa60.9BOPT are satisfied.
According to the eleventh aspect of the present invention, in the eighth aspect, BX=1 is satisfied.
According to the twelfth aspect of the present invention, in the seventh, eighth, ninth or tenth aspect, 20xe2x89xa6Mxe2x89xa680, L=6 and D=2 are satisfied.
According to the thirteenth aspect of the present invention, in the seventh, eighth, ninth or tenth aspect, 40xe2x89xa6Mxe2x89xa6100 and BXxe2x89xa60.7BOPT are satisfied.
According to the fourteenth aspect of the present invention, in the first, second, third, fourth, fifth, seventh, eighth, ninth, tenth or eleventh aspect, 24xe2x89xa6Mxe2x89xa640 and BXxe2x89xa60.75BOPT are satisfied.
In accordance with the present invention, a multiple line addressing driving is effected with an L number of simultaneously selected row electrodes to provide Lxe2x89xa0{square root over (M)} where M represents the total number of row electrodes for driving a display area and L represents the number of simultaneously selected row electrodes, wherein driving is performed at a driving bias ratio which is deviated toward the minimum bias ratio at which a driving voltage is the minimum at VR/VC=1 with respect to the optimum bias ratio at which an ON/OFF ratio is the maximum. Accordingly, a low driving voltage is obtainable in comparison with the conventional case where driving is effected by MLA system with L={square root over (M)} while the reduction of picture quality is prevented and a practical ON/OFF ratio can be maintained. Thus, a low voltage driving is performed, which was impossible in the conventional driving system.
Further, in the present invention, since driving is performed with the minimum bias ratio (VR/VC)=1, a part of column voltage levels and a voltage level applied to row electrodes can commonly be used, and the number of voltage levels necessary to drive the liquid crystal can be reduced. With this, the power source voltage circuit to generate voltage levels can be simplified and, cost reduction and low power consumption are obtainable.
In particular, when dummy rows are used, a multiple line addressing driving is effected with an L number of simultaneously selected row electrodes to provide {square root over ((M/Lxc2x7(L+D)))}xe2x89xa0N where D represents the number of dummy rows and N represents the maximum magnifying power of a unit column voltage obtained by a predetermined matrix calculation based on display data and scanning voltages applied to the row electrodes. In this case, when the driving is performed at a bias ratio of VR/VC which is deviated toward the minimum bias ratio at which a driving voltage is the minimum with respect to the optimum bias ratio at which the ON/OFF ratio is the maximum, the same effect can be expected. Further, in this case, with use of dummy data, the relation between image data and column voltage can be determined so as not to use a column voltage series including the maximum column voltage. In this case, when selection is repeated on the same simultaneously selected number L, the effect for lowering driving voltage is obtainable in comparison with a case without using dummy rows. Further, from the above-mentioned reason, the number of column voltage levels can be reduced, and accordingly, the power source voltage circuit can be simplified, and reduction of cost and low power consumption can be realized.
Further, since the driving is performed with an L number of simultaneously selected rows which satisfies 0.3{square root over (M)}xe2x89xa6Lxe2x89xa62{square root over (M)}, and at a bias ratio in a range of from 0.3 times to 0.9 times as much as the optimum bias ratio, a predetermined ON/OFF ratio can be assured regardless of the total number of electrodes for the display area, and driving is performed with a low driving voltage which was impossible to realize in the conventional MLA system.
In particular, when the total number of row electrodes for the display area is 20-100, and driving is effected with a simultaneously selected number of 4 at the minimum bias ratio (VR/VC)=1, the number of voltage levels necessary to drive the liquid crystal can be reduced in addition to the effect for lowering voltage; the power source voltage circuit for generating voltage levels is simplified, and reduction of cost and low power consumption are obtainable.
Further, in use of dummy rows, a number of dummy rows satisfying D/(D+L) less than 0.5 should be used for driving. There tends to decrease the ON/OFF ratio with increasing the number of dummy rows in a case of driving using a same simultaneously selected number and a same bias ratio. However, with such range, there is no possibility of being recognized as reduction of display quality. Further, the lowering of voltage is obtainable.
In particular, when driving is performed with a total number of row electrodes of display area of 20-80, a simultaneously selected number of 6 and a dummy row number of 2, the column voltage level can be reduced in addition to remarkable effect of lowering voltage, whereby the power source voltage circuit can be simplified. Further, reduction of cost and low power consumption are realized.
On the basis of the above-mentioned inventions, when driving is performed under the conditions that the total number of row electrodes for the display area is 40-100, and a bias ratio which is not more than 0.7 times as much as the optimum bias ratio is used for driving, it is sufficient to drive with a power source voltage of 5.5 V or lower. Accordingly, unlike the conventional driving method, it is possible to form a power source with a voltage increased by twice even when a button battery is used for driving, and a special reduction of power consumption is obtainable. Further, since a ratio of the power source voltage to the maximum value of the column voltage and row voltage supplied to the liquid crystal is small, a driving circuit for the liquid crystal device can be formed with a standard logic process.
Further, when driving is performed under the conditions that the number of the total number of row electrodes for the display area is 40-100, and a bias ratio which is not more than 0.6 times as much as the optimum bias ratio is used for driving, it is sufficient to drive with a power source voltage of 5.0 V or lower. Accordingly, a sufficient margin can be assured even when there is a temperature variation. Therefore, a standard logic process can be used to form a driving circuit for the liquid crystal display device so as to obtain a stable operation in addition to the permission of use of a button battery for driving.
Further, when driving is performed under the conditions that the total number of row electrodes for the display area is 24-40 and a bias ratio which is not more than 0.75 times as much as the optimum bias ratio is used for driving, it is sufficient to drive with a driving voltage of 3.3 V or lower. Accordingly, a button battery can be used for directly driving, and the structure of the device can be simplified by, for example, eliminating a voltage increasing circuit.